US 7221209 B2 Abstract A circuit and corresponding method for a precision floating gate voltage reference that uses a feedback loop, conduction of tunnel devices, and a bandgap cell to accurately program a desired charge level on a floating gate and provide a predictable and programmable temperature coefficient parameter for such voltage reference. In one embodiment, a bandgap cell is coupled through a capacitor to the floating gate storage node for providing a voltage source for canceling the temperature coefficient (TC) of the storage capacitor. The circuit and method enables TC to be minimized by either choosing the proper voltage source characteristics or alternatively, by choosing the proper ratio of two capacitors. The bandgap cell can alternatively be designed to have positive TC (PTAT voltage sources) or negative TC (VBE junction).
Claims(24) 1. In a floating gate voltage reference circuit having a floating gate for storing charge thereon, said charge appearing at a node coupled to an input of an opamp, wherein a voltage reference output is generated at the output of the opamp as a function of the charge on said floating gate, said circuit also having a first capacitor coupled to said node; a method for improving the accuracy of the voltage reference output of said circuit as a function of temperature, comprising:
supplying a voltage source providing an output having a predetermined and substantially constant Temperature Coefficient (TC); and
coupling a second capacitor in series between said voltage source and said node so as to compensate for the TC of said first capacitor; wherein said first capacitor and said second capacitor are the same type of capacitor.
2. The method of
3. The method of
4. The method of
5. The method of
6. The method of
7. The method of
8. The method of
9. The method of
10. The method of
11. The method of
12. A floating gate reference circuit for improving the accuracy of a voltage reference output of said circuit as a function of temperature comprising:
a floating gate for storing charge thereon, said charge appearing at a node coupled to an input of an opamp, wherein a voltage reference output is generated at the output of the opamp as a function of the charge on said floating gate;
a first capacitor coupled to said node;
a voltage source providing an output voltage having a predetermined and substantially constant TC; and
a second capacitor connected in series between said voltage source and said node so as to compensate for the TC of said first capacitor; wherein said first capacitor and said second capacitor are the same type.
13. The reference circuit of
14. The reference circuit of
15. The reference circuit of
16. The reference circuit of
17. The reference circuit of
18. The reference circuit of
19. The reference circuit of
20. The reference circuit of
21. The reference circuit of
22. The reference circuit of
23. In a floating gate voltage reference circuit having a floating gate for storing charge thereon, said charge appearing at a node coupled to a first input of an opamp, wherein a voltage reference output is generated at the output of the opamp as a function of the charge on said floating gate, said circuit also having a first capacitor coupled to said node; a method for improving the accuracy of the voltage reference output of said circuit as a function of temperature, comprising:
coupling a second capacitor to a second input of said opamp; wherein said first capacitor and said second capacitor are the same type of capacitor;
supplying a voltage source providing an output having a predetermined and substantially constant Temperature Coefficient (TC); and
connecting said voltage source in series with said second capacitor so as to compensate for the TC of said first capacitor.
24. The method of
Description The invention relates generally to the field of circuit design and in particular to improving the accuracy of a floating gate voltage reference circuit. One of the key performance parameters for precision voltage references and comparators is the temperature coefficient (TC). The TC parameter specifies the amount of voltage change which occurs as a result of a change in temperature. TC for a given component may be positive, negative, or may change direction over various temperature ranges. The bandgap and buried zener are two known methods for implementing voltage references. The bandgap and buried zener voltage references utilize special bipolar or BiCMOS process technologies. These types of references require various trimming methods, e.g., laser trimmed thin-film resistors or metal fuses, for achieving close to 1 mV initial accuracy and a TC at or below 5 ppm per degree C. More recently, a precision floating gate voltage reference (FGREF) has been implemented on EEPROM CMOS technology. A precision floating gate voltage reference stores a known voltage on a floating capacitor tied to the input of an opamp. Tunnel diodes are typically used as switches to charge the floating capacitor during the programming (set) mode. The TC of the FGREF depends on the TC of the storage capacitor. In order to achieve close to zero TC, known circuits and methods utilize a mix of different types of capacitors for causing the composite TC of the capacitors to be near zero. The temperature coefficient of voltage reference circuit 10 is a function of the TC of the capacitor C. The TC of capacitor C is typically fairly low (˜+20 ppm/C) for Poly1/Poly2 capacitors in CMOS technology. Since the storage node 11 is floating and fully protected from any outside or inside contact, charge conservation principles can be applied to calculate the TC of Vout due to the change in the value of Capacitor C with temperature. A set of Equations 1 below shows that TC of Vout is the negative of the TC of the capacitor C. EQUATIONS 1: Charge at Storage Node 11 is given by Q(t_{0})=constant, determined at programming time and a selected temperature, t_{0}.
Since the TC of Vout is the negative of the TC of the capacitor C, in order to get zero TC at Vout, capacitors with near-zero TC are required. In one known method, two different types of capacitors are combined for minimizing TC. EQUATIONS 2: Where t=die Temperature, t_{0}=ambient temperature during the programming of the voltage reference circuit, Δt=t−t_{0}, α=TC of a CP type capacitor, and β=TC of a CPD type capacitor:
In As shown in What is therefore needed is a method for TC cancellation for a floating gate voltage reference that uses only one type of capacitor so as to provide a predictable and programmable TC for the overall voltage reference generator circuit. What is also needed is an analog floating gate voltage reference circuit for accurately programming a desired charge level on a floating gate and for making TC reduction methods more reliable and repeatable for different output voltage values. The present invention overcomes the drawbacks of known circuits and methods by providing a circuit and method for minimizing TC more reliably in a high precision floating gate reference. The circuit and corresponding method of the present invention uses only one type of capacitor so as to provide a predictable as well as a programmable TC for such references. In one embodiment according to the present invention, a bandgap cell is coupled through a capacitor to the storage node in order to cancel the TC of the storage capacitor, wherein both capacitors are of the same type. The bandgap cell can be designed to have Positive TC (Proportional to Absolute Temperature (PTAT) source) or Negative TC (Voltage Base-Emitter (VBE) junction source). An advantage of the present invention is that the TC of a PTAT or VBE source is very reliable and nearly process/technology independent. As a result, a more predictable and programmable TC of the overall FGREF is provided. Standard CMOS technology has only one type of capacitor element. Thus, another advantage of the present invention is that it enables minimizing TC in a high precision floating gate voltage reference circuit utilizing standard CMOS technology. Another advantage of the present invention is that it makes minimizing TC more predictable. In an alternative embodiment, a predictable TC value can be dialed in via a programmable control register. Broadly stated, the present invention provides, in a floating gate voltage reference circuit for storing a predetermined voltage at a first node coupled to an input of an opamp wherein a voltage reference output is generated at the output of the opamp as a function of the charge of the floating gate, the reference circuit having a first capacitor coupled to the first node; a method for improving the accuracy of the voltage reference output as a function of temperature, comprising coupling a second capacitor to an input of the opamp; wherein the first capacitor and the second capacitor are the same type of capacitor; supplying a voltage source providing an output having a predetermined and substantially constant Temperature Coefficient (TC); and connecting the voltage source in series combination with the second capacitor so as to compensate for the TC of the first capacitor such that, during a read mode of the reference circuit, the temperature coefficient, TC, of the voltage reference output is substantially reduced. Broadly stated, the present invention also provides a floating gate reference circuit for improving the accuracy of a voltage reference output as a function of temperature comprising a floating gate for storing charge thereon, the charge appearing at a first node coupled to an input of an opamp, wherein a voltage reference output is generated at the output of the opamp as a function of the charge of the floating gate, a first capacitor coupled to the first node; a second capacitor coupled to an input of the opamp; wherein the first capacitor and the second capacitor are the same type; and a voltage source providing an output voltage having a predetermined and substantially constant TC; the voltage source connected in series combination with the second capacitor so as to compensate for the TC of the first capacitor such that, during a read mode of the reference circuit, the TC of the voltage reference output is substantially reduced. These and other embodiments, features, aspects, and advantages of the invention will become better understood with regard to the following description, appended claims and accompanying drawings. The foregoing aspects and the attendant advantages of the present invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: Reference symbols or names are used in the Figures to indicate certain components, aspects or features shown therein, with reference symbols common to more than one Figure indicating like components, aspects or features shown therein. The present invention is a system and method for improving the accuracy of the output reference voltage (V_{ref}) of a floating gate voltage reference circuit as a function of temperature. An object of the present invention is to minimize Tc in a high precision floating gate voltage reference circuit in a more predictable and programmable way. EQUATIONS 3: Where t=die Temperature, t_{0}=ambient temperature, i.e., 25° C., capacitors C_{0 }and C_{1 }are the same type of capacitors with the same TC=α:
Circuit 410 includes MOSFET transistors M_{0}, M_{1}, M_{2}, M_{3}, M_{4}, and M_{5}, PNP transistors Q_{1}, Q_{2}, and Q_{3}, resistor R_{0}, variable resistor R_{1}, and a 4:16 decoder. Transistors M_{0}, M_{1}, M_{2}, and M_{3 }are connected so as to provide a current mirror that causes the current in transistors Q1 and Q2 to be either equal or an exact multiple of each other. For simplification of the description, it is assumed that transistor Q_{1 }and transistor Q_{2 }conduct the same amount of current. The size of the emitter area for transistor Q_{2 }is ten times, i.e., 10×, the size for Q1, i.e., 1×. As a result, the base-emitter voltage of transistor Q_{2}, V_{BE2}, will be smaller than the base-emitter voltage of Q_{1}, V_{BE1}. The difference between the base-emitter voltages of transistors Q_{1 }and Q_{2 }is in accordance with the equation: ΔV_{BE}=V_{BE2}−V_{BE1}=(kT/q)ln(10), where 10 is the ratio of the two emitter areas, k is Boltzmann's constant, and q is the electron charge. The voltages across transistor M_{0 }and M_{1 }are the same since it was assumed that the transistor Q_{1 }and transistor Q_{2 }conduct the same amount of current. This causes the voltage across resistor R_{0 }to equal (kT/q)ln(10). The corresponding current for R_{0}=V_{BE}/R_{0}=(kT/R_{0}q)ln(10) which flows through transistor M_{3}. The current through M_{4 }is the same as the current for M_{3 }and is referred to as PTAT since the current is Proportional To Absolute Temperature in accordance with (kT/R_{0}q)ln(10). In circuit 410, the current flowing through variable resistor R_{1 }creates a voltage V_{p }as a function of the resistance set for variable resistor, R_{1 }via the 4 to 16 decoder. Vp is the voltage across R_{1 }and is given by V_{P}=αR_{1}/R_{0}*(kT/q)ln(10), where αR1 is the resistance set for variable resistance R1 via the 4 to 16 decoder. Another sample of the current from transistor M_{3}, i.e., I_{ptat }is forced to conduct from transistor M_{5}. A current I_{ptat }also flows through transistor Q_{3 }and creates a voltage V_{B}. The voltage V_{B }is the base-emitter voltage of transistor Q_{3 }since the base of Q3 is connected to ground. The temperature of a base-emitter junction of PNP transistor Q3 is known to vary by approximately −2 mv/° C. or 3000 ppm/° C. over a very broad temperature range. For circuit 600, in order to adjust TC of reference voltage, V0, either the magnitude of V_{B }or the magnitude of C_{f1 }can be adjusted. Referring to The present invention according to the embodiment in The exemplary circuit 410 in Equations 4 show that, for a particular V_{B }value, by choosing a proper ratio of C_{f1}/C_{f0 }or V_{B0}, TC can be minimized. EQUATIONS #4: For V_{B}(t) such that: According to an alternative embodiment of the present invention, the voltage source for the voltage reference of the present invention may also be provided by another floating gate reference. As described above, the present invention minimizes TC more reliably in a high precision floating gate reference. The circuit and corresponding method of the present invention uses only one type of capacitor so as to provide a predictable as well as programmable TC for such references. Although specific embodiments of the invention have been described, various modifications, alterations, alternative constructions, and equivalents are also encompassed within the scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that additions, subtractions, deletions, and other modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims. Patent Citations
Non-Patent Citations
Referenced by
Classifications
Legal Events
Rotate |